Etch processes in state of the art semiconductor technology use endpoint detection techniques to avoid excessive overetching. These techniques have become more sophisticated as design rules shrink and greater control over etch parameters becomes necessary.
A widely used end point detection technique is to detect chemical changes in the etching environment. Monitoring the atmosphere in an etch reactor by spectroscopy reveals when one layer has cleared and another has begun to etch. However, when very precise control is required the detection of this endpoint actually occurs when overetching has already begun. Typically, the etch profile in a window near the termination of the etch step is "dished", with the center portion cleared but with material still remaining at the edge of the window. Normal practice is to overetch, to clear the entire window. This causes the underlying layer to be etched. While slight overetching has been acceptable in many prior art processes, new device structures may not tolerate overetching. For example, gate dielectrics in current ULSI technology may be tens of Angstroms. If a polysilicon gate is patterned over such a thin gate dielectric using prior art techniques, some portions thereof may be thinned excessively, or even cleared to the substrate.
The generic problem with chemical end point sensors is that they infer the layer dimensions. Non-destructive techniques for measuring the layer thickness directly, which can be used for in-situ monitoring, are preferred because the thickness of the layer being etched is not only directly measured, but the endpoint can be anticipated and stopped before clearing of the next layer begins. With this capability, a two step etch can be performed. In the first etch step an aggressive etch can be used. When onset of clearing is imminent, the etch chemistry can be switched to a highly selective etchant. In this way a squared etch profile can be produced with essentially no overetching. As is known, etch selectivity is defined as the ratio of the rate of etching of said layer being etched to the rate of etching of the layer underneath the layer being etched. A highly selective etch in state of the art plasma processing can be defined as an etch with a selectivity of greater than 40:1.
Optical end point detection techniques have been proposed which measure layer thicknesses directly. For example, polarization ellipsometry is a very sensitive end point detection technique. See U.S. Pat. No. 5,494,697, issued Feb. 27, 1996. While this technique is effective, it is complex and requires expensive tools.
Interferometric end point detection schemes have also been proposed. These rely on the detection of optical fringes from a reflected light beam. The pattern of fringes changes as the layer thickness changes. The fringes cease when the layer thickness is zero. However, here again the end point cannot be anticipated, and precise end point detection is difficult to realize. As in the case of chemical end point detection, prior art interferometric detection schemes also produce some degree of overetching.
Improved endpoint detection techniques are continually sought which provide sensitive and direct thickness measurements, are compatible with dry etch processes, and can be implemented as in-situ monitoring tools.